Best practice and method for transparency with leaves?
Translucent BSDF is physically more acurate in this situation.  


Connect it using Mix Shader with Principled BSDF
Use the Fac slider to control the influence
Use Cycles render engine for realistic translucence (Eevee doesn't support it)




If you want to use only Principled BSDF shader, use Transmission + Transmission Roughness. Notice the light scattering isn't as strong.



Subsurface Scattering doesn't directly affect translucency, but it's needed for proper realism. Just bear in mind it's a bit slower.